The present invention relates to a light-weight magnetic suspension momentum device which is easy to manufacture.
In space applications there is a need for momentum devices which are as light and reliable as possible consistent with acceptable performances and also it is desirable to have devices with as simple a design as possible.
Generally the momentum devices have a stator member which is fixed to the spacecraft and a rotor member which provides the required momentum when it rotates at a nominal speed relative to the stator members. An advantageous type of momentum device is that one having a contactless magnetic bearing system for supporting the rotor member. The known devices in this category are generally mechanically complex with significant mass for the stator member.
An improved design is a two-degree of freedom active bearing device as disclosed in U.S. Pat. No. 4,000,929. This device comprises a stator member in the form of a disc and an annular rotor member situated in the same plane as the disc and separated therefrom by a radial airgap in which a radial magnetic field is produced for supporting the rotor member. This suspension magnetic field is produced by a permanent magnet centrally located on the stator disc. When the disc has a small thickness and four curved polar faces confronting the inner face of the annular rotor and when the latter rotates at high speed relative to the stator, the magnetic flux in the airgap reverses at 90 degree angular intervals in the rotation plane, which causes high eddy current losses to occur whereby the free rotation of the rotor is prevented. This drawback is overcome when the stator member consists of a double disc such that its polar faces are displaced in the axial direction and are substantially aligned in parallel planes thereby to reduce the magnetic flux path through the annular rotor. Such an embodiment, however, is disadvantageous in practice as it reduces the rotor mass at the expense of increased stator mass.
Now, on a spacecraft it is very important to reduce the stator mass as low as possible so as to enable the device to have a higher payload capability. Also, the permanent magnet centrally located on the stator has a relatively small volume which leads to a poor transverse stiffness capability when this device is used as a momentum device. If in this device the volume of the permanent magnet is increased in order to obtain an acceptable transverse stiffness, the stator mass is also substantially and harmfully increased. In this respect too, the device disclosed in the U.S. patent referred to above is inadequate, since the transverse stiffness is of prime importance for momentum wheel applications and especially for systems operating at high speeds and having high momentum values.
The U.S. Pat. No. 4,043,614 discloses another device of the same type as described above. The stator and rotor members comprise an annular permanent magnet on the opposed faces of which are fixed two parallel thin polar rings of a magnetic permeable material. The polar rings on the stator and rotor members extend in two parallel planes. The polar rings on the stator member have cut-outs to accomodate sectoral electrical coils for creating a magnetic induction in the adjacent magnetic permeable rings on the rotor member.
A major drawback in this embodiment is that the stator member does not have any magnetic permeable path between the polar rings carrying the coils, which limits the magnetic suspension rigidity. Furthermore, the problem with this embodiment resides in that the coils on the stator are difficult to fabricate and to assemble. Also, the presence of the cut-outs in the polar rings for accomodating the coils renders these polar rings insufficiently rigid and consequently, to render these polar rings sufficiently stiff, it is necessary to provide the stator with sufficient stiffener means which increase the stator mass. In addition, the presence of the cut-outs causes non negligible eddy current to produce rotation losses.